1. Field of the Invention
The present invention relates to a high temperature/high pressure process for preparing a solution of alkali metal phosphate from a selected ammonium phosphate while simultaneously recovering ammonia from the reaction mixture.
2. Description of the Prior Art
In the prior art, alkali metal phosphates such as monosodium phosphate, disodium phosphate, and trisodium phosphate, solutions thereof and compounds derived therefrom have been commercially prepared by neutralizing of furnace grade or wet process phosphoric acid at atmospheric pressure with an alkali metal hydroxide or carbonate such as sodium or potassium hydroxide or carbonate and then further treating the product to form a desired phosphate, polyphosphate, or pyrophosphate.
In such processes an ammonium phosphate is not formed as an intermediate for at least two reasons. Ammonia required for the preparation of an ammonium phosphate intermediate is costly and generally not readily available at the phosphate production site. Secondly, an economical process for utilizing a route involving an ammonium phosphate intermediate would require an efficient, inexpensive method for recovering ammonia for sale or recycle to the process and a suitable ammonia recovery process was not heretofore available.
It is generally known that the degree to which ammonia can be recovered from an aqueous system is a function of the alkalinity of the system. Thus, in the Kjeldahl method for analyzing for ammonia, highly alkaline solutions are employed in order to facilitate the release of ammonia. Likewise, in urea technology use of highly alkaline solutions facilitates substantially complete recovery of ammonia.
In aqueous phosphatic systems the ability to remove ammonia is also dependent on the alkalinity of the solution. Thus at atmospheric pressure complete recovery of ammonia from an ammonium phosphate solution is only feasible in the presence of a high concentration of a base such as sodium hydroxide, for example, in amounts sufficient to provide an Na/P molar ratio of 3 or more. As one decreases the concentration of base, recovery of ammonia becomes less complete and ammonia recovery times increase dramatically. As the Na/P molar ratio is decreased from 3 or more to about 2 or less ammonia recovery becomes uneconomical due to low yields and extended recovery time.
It is self-defeating to utilize a high Na/P molar ratio to facilitate ammonia recovery during alkali metal phosphate formation due to the fact that the high Na/P molar ratio merely limits the variety of phosphates which can be ultimately formed. For example, if an Na/P molar ratio of 2 or more is utilized to form an orthophosphate solution from an ammonium phosphate, the orthophosphate cannot advantageously be used to form such relatively acidic products as NaH.sub.2 PO.sub.4, Na.sub.2 H.sub.2 P.sub.2 O.sub.7 and (NaPO.sub.3).sub.x since each require an Na/P molar ratio of 1. Likewise, an orthophosphate solution having an Na/P molar ratio of 2 or more cannot advantageously be utilized as a source material for sodium tripolyphosphate which requires an Na/P molar ratio of about 1.667.
It is thus apparent that if substantially all ammonia could be recovered from relatively acidic ammonium phosphate solutions, for example, those having an alkali metal/P molar ratio below about 2 and preferably as low as about 1 the resulting orthophosphate could advantageously be employed to prepare all phosphates, polyphosphates, and pyrophosphates in which the ratio of alkali metal to P is 1 or more.
Thus, the principal object of the present invention is to provide a method for preparing from a selected ammonium phosphate an alkali metal orthophosphate solution which has a low alkali metal/P molar ratio and for simulteneously recovering the ammonia present in the starting ammonium phosphate.
It is also an object of the present invention to provide such a process in which substantially all such ammonia may be recovered, even at low alkali metal/P molar ratios.
It is a further object of the present invention to provide a process which is sufficiently versatile as to be capable of economically producing ammonia of any desired degree of purity and a solution of an alkali metal orthophosphate having any desired alkali metal/P molar ratio in the range of 1-2.